The present invention relates generally to electrocatalysis, and more particularly to unique metal-oxide electrocatalysts and their use in reduction processes for producing amines from nitriles.
Hydrogenation of nitriles to amines has long enjoyed substantial academic and commercial interest. It is recognized that these hydrogenations require a catalyst to promote reduction for best efficiency. On all scales, and particularly on a commercial scale, most such reductions are conducted by simply passing chemical reactants of choice, usually substrate plus hydrogen, over catalysts at elevated temperatures and pressures. This has generally been termed a "secular" type reaction. For example, Raney nickel is one of the most widely used catalysts for secular hydrogenations to produce amines from nitriles. As is well known, Raney nickel is a high surface area material derived by leaching the aluminum from a nickel/aluminum alloy. The resulting nickel material, in spite of its recognized pyrophoric nature, has become a hydrogenation catalyst of choice.
For example, much work has focussed upon Raney nickel catalyzed hydrogenations of adiponitrile (ADN) to hexamethylenediamine (HMD). HMD is a large volume, highly useful chemical, notably in large amounts as a Nylon 6,6 intermediate. The secular catalytic hydrogenations used today in large scale HMD production require high pressures, high temperatures, and an expensive (usually Raney Ni) catalyst. Moreover, these secular hydrogenations have been known to form undesirable by-products removable only with great difficulty and expense, if at all. Often, these by-products significantly interfere with the use of the formed amine. For instance, Raney Ni catalyzed reduction of ADN to HMD is known to form 1,2-diaminocyclohexane (DAC), which is not readily separated from the HMD product. This DAC by-product is then included in the HMD during processing to form Nylon, which, as those in industry know, leads to highly undesirable discoloration of the Nylon. Nonetheless, as already stated, much work still focuses on such secular catalytic processes, with more recent work emphasizing process refinements. See, U.S. Pat. Nos. 3,821,305; 4,247,481; and 4,359,585. In addition to the nickel catalysts, Raney cobalt borohydride has been reported as a heterogeneous catalyst for secular catalytic hydrogenations of ADN to HMD. See, Japanese patent No. 7511138 (1979).
Despite many potential advantages to be gained, the study of electrocatalytic (i.e. cathodically catalyzed) approaches to hydrogenating nitriles has generally failed to keep pace with that of secular-type reactions. There are relatively few such electrocatalytic studies reported, most originating from the U.S.S.R. As an example, early on, A. P. Tomilov developed and reported an electrochemical process for direct reduction of ADN to HMD. Tomilov, A.P., "Electrochemical Synthesis of Hexamethylenediamine and Aminocaponitrile", Khim. Prom. 329-333 (1965); see also Soviet Patents 137,924 (1961) and 445,647 (1974) (cited in Kuhn, A. T., "Industrial Electrochemical Processes", Elsevier Pubs., New York, pp. 609-610 (1971)). This original process utilized copper sponge-covered steel metal cathodes and magnetite anodes. A sodium hydroxide electrolyte was used at a 20-50 cm/sec electrolyte velocity, which was needed to maintain ADN as an emulsion. Current density was 60 mA/cm2. Under these conditions, a total current efficiency of 51% was reportedly obtained, with the product ratio being 4.4:5.6, HMD:Aminocaponitrile, or 44% HMD.
In later work, Tomilov changed the process to include a diaphragm and paired production of chlorine at the anode. Accordingly, in this newer work the anolyte was 25-30% HCL, and the catholyte 96.9 g/L ADN in NaOH. Further, a nickel metal sponge cathode was used, as was a current density of 100 mA/cm.sup.2. Under these conditions, the reported total yield improved from 51% to 72%, with the yielded product being 60% HMD and 12% aminocaponitrile (ACN). It was not reported why better yields obtained in the later work. However, it was claimed that the process could produce HMD at 20% lower cost than secular-type catalytic hydrogenation. Rounding out this work in the U.S.S.R. is a "Deposited Document" which relates to optimization of an electrocatalytic process for reducing ADN, this time using a Raney iron electrocatalyst. Andriyanaova, I. P. et al., "Optimization of Electrocatalytic Reduction of Adiponitrile on Raney Iron", Deposited Doc. Khim. Metal Inst., Karaganda USSR (1980).
Despite this work in the U.S.S.R. and Tomilov's claim of low cost HMD production, to applicant's knowledge, no electrocatalytic route to HMD as been adopted commercially. This may be because the secular catalytic hydrogenation processes have been highly optimized, and any displacement of them commercially will have to come from an electrocatalytic process achieving markedly high efficiency. Such an electrocatalytic process would offer significant advantage over secular processes in providing low cost routes to HMD and other amines, but, up to now, no such process appears to have demonstrated the requisite efficacy. The applicant entered a study in light of this background, and has now discovered unique cathodic electrocatalysts which provide preferred processes of surprisingly high efficiency.